1. Field of the Invention
The present invention relates to thin film capacitor devices or ferroelectric devices and, more particularly, to a method of forming an electrode on a glass layer of a substrate.
2. Brief Description of Prior Developments
Platinum (Pt) is commonly used as an electrode material for thin-film ferroelectric devices. This is due to its resistance to oxidation at temperatures above 600° C. in oxygen (O2); an environment which is required to obtain good electrical characteristics from materials such as (BaSr)TiO3 (BST), (PbZr)TiO3 (PZT), SrBi2Ta2O9 (SBT), SrBi2Nb2O9 (SBN), SrBi2(TaNb)2O9 (SBTN) and others. Devices using these materials are frequently made using oxidized silicon wafers as a substrate.
Adhesion of a platinum electrode to the oxide of the silicon wafer has always been a problem. The problem has been exacerbated due to the unique properties of platinum and challenges of thin film ferroelectric processing. Platinum has a tendency to plastic flow in response to stress at temperatures above 600° C. Ferroelectric thin film deposition is frequently performed using spin-on methods which result in a very high tensile stress due to film shrinkage as it is sintered. The ferroelectric material can contain metals such as Bi. These metals can alloy with the platinum.
The ferroelectric processes often incorporate long anneals in O2 at elevated temperature to improve performance. Even if the platinum adheres to the substrate, these processes frequently cause the formation of hillocks or bumps on the surface of the platinum electrode. These hillocks can cause localized high electric fields, high leakage, and early breakdown. In more severe cases, hillocks can be large enough to directly short out the ferroelectric device or combine with other defects in the film to short out the devices resulting in reduced yield or unusable devices.
Historically a thin film of titanium (Ti) has been added between the platinum and the oxide of the silicon wafer. This titanium layer is about 10% of the thickness of the platinum electrode and greatly improves adhesion of the platinum to the oxide. An example of this electrode is given in U.S. Pat. No. 5,723,171. The use of a titanium adhesion layer has two major problems. First, the process windows are very narrow. Changes in anneal times or temperatures or changes in the thickness of the ferroelectric often require a re-optimization of the titanium and platinum layer thicknesses. Second, titanium is very mobile and can migrate through the platinum electrode causing degraded performance of the ferroelectric layer. This is a known problem with BST, SBT, SBTN and others where variations in film composition, due to titanium incorporation degrade film performance.
Other methods to improve adhesion have been employed with a wide variety of metallic layers such as Cr, Ta, Vd, Nb, Sr, Ru, Os, Pd (see U.S. Pat. Nos. 6,103,400 and 6,054,311). While these other metals may work over some range of conditions, these processes all suffer from either poor adhesion or from volume expansion of the adhesion layer due to oxidation during high temperature oxygen anneals which results in an unstable foundation for the platinum and limits their utility.